1. Field of the Invention
The present invention relates to a catalyst composition for preparing carbon nanotube containing multi-component support materials of amorphous Si, Mg and Al as well as a bulk scale preparation process for preparing carbon nanotube using said catalyst composition. More specifically, this invention relates to a process for preparing carbon nanotube using the catalyst composition comprising a transition metal catalyst and support materials of amorphous Si, Mg and Al. Further, the present invention also provides a bulk scale preparation method for preparing carbon nanotube having the following characteristics: the diameter of carbon nanotube is 5˜20 nm; the number of wall is less than 15; the apparent density of carbon nanotube is 0.03˜0.08 g/cc; and the carbon conversion yield is in the range of 40˜80%.
2. Description of Prior Art
Carbon nanotube was firstly disclosed by Dr. Iijima in Nippon Electric Company (NEC) by arc discharging the carbon rod containing metal catalyst (S. Iijima, Nature, 354, 56 (1991)). The further studies of carbon nanotube disclosed that carbon nanotube shows diverse and advantageous physical and chemical properties. The technical developments in controlling the structure of carbon nanotube let it be applied to various fields of industries, such as, reinforcing agent of polymer, pharmaceuticals, storage of energy, catalyst support for polymer synthesis.
A research group of Baker and N. M. Rodriguez in the United States has specifically developed the crystalline structure of carbon nano materials (J. Mater. Res., Vol 8: 3233˜3250, 1993). As preparation methods of carbon nanotube, an arc discharge method, a laser ablation method, a catalytic growing method and a plasma method have been described in following documents: that are, R. E. Smalley et al., J. Phs. Chem., 243, 49 (1995); M. Endo et al., Carbon, 33, 873 (1995); U.S. Pat. No. 5,424,054; Chem. Phys. Lett., 243, 1-12 (1995); Science, 273: 483-487 (1996); and U.S. Pat. No. 6,210,800.
For a commercial use of carbon nanotube, it is very important to produce high quality of carbon nanotube in a low cost. It has been known that structural control of diameter or length of carbon nano material can be accomplished by understanding of transition metal, catalyst support materials and interaction between transition metal and catalyst support materials.
In PCT International publication No. WO 2006/50903 ‘Catalyst for producing carbon nanotubes by means of the decomposition of gaseous carbon compounds on a heterogeneous catalyst’, it has been disclosed that the transition metal catalyst composition comprising Mn, Co, optionally Mo and a support material enables to produce carbon nanotube having 3˜150 nm of diameter in a high catalytic yield. However, there is no specific description about the role of catalyst support materials.
In Korean Early Patent Publication No. 10-2004-82950 ‘Synthetic method of large amount of double walled carbon nanotube by chemical vapor deposition method’, the single walled or double walled carbon nanotube prepared by a catalyst complex has been disclosed. In this disclosure, the catalyst complex has been prepared by inserting the transition metals, such as, Fe, Co, Ni, Mo or alloy of them into the nano scale pore in support materials consisting of MgO, alumina, zeolite or silica. However, such techniques definitely have their limitation to the commercialization because it is difficult to prepare 2˜5 nm of nano-sized Fe, Co, Ni and/or Mo transition metals as catalyst metal particles in bulk.
In Korean Early Patent Publication No. 10-2006-18472 ‘Process for preparing carbon nanotube using the mechano-chemical treated catalyst’, carbon nano fiber prepared by a chemical vapor deposition method using acetylene as a carbon source in the presence of mechano-chemical treated support catalyst comprising Ni and Mg support has been disclosed. However, Mg support had been already disclosed before this invention. Further, there is no specific description regarding the function of support materials.
In Korean Early Patent Publication No. 10-2005-78596 ‘Purification method of carbon nanotube and preparation method of carbon nanotube’, carbon nanotube prepared by a plasma chemical vapor deposition method has been disclosed. Further, in this preparation method, the plasma chemical vapor deposition method comprises i) substrate preparation step for growing carbon nanotube; ii) growing step for carbon nanotube on the said substrate; and iii) purification step of carbon nanotube using plasma of inert gas has been disclosed. As a catalyst composition, the mixed metal composition of Ni, Fe and Co has been disclosed. However, the transition metal as well as support material used in this disclosure has been already disclosed. Further, there is no description about the advantageous effect of catalyst in itself and support materials.
The preparation method for the synthesis of carbon nanotube disclosed in various technical documents or patents can be specified by the kind and ratio of transition metals and the shape and size of support materials included in catalyst composition. Regarding the preparation of catalyst composition, it has been described in following documents. P. E. Anderson et al., J. Mater. Res., 14(7), 2912 (1999); and R. J. Best, W. W. Russell, J. Am. Chem. Soc., 76, 838 (1974). Nonetheless, it is still required to develop a catalyst composition to enhance the catalytic productivity as well as to realize structural characteristics of carbon nanotube by controlling the numerous variables regarding catalyst synthesis.
Only a few documents disclosed the characteristics of catalyst suitable for the continuous process for the synthesis of carbon nanotube, while most of them disclosed the characteristics of catalyst suitable for the batch process for the synthesis of carbon nanotube.
Transition metal and support material in the catalyst composition exist in the form of multi-oxide complex due to the heat treatment of them over 600° C. under atmosphere. During this heat treatment step, the catalyst particles become to be grown. It has been known that the size of catalyst particles plays an important role in determining the diameter of carbon nanotube.
Ultimately, it is important to obtain the homogeneity of a catalyst composition under the precise heat treatment conditions in order to produce a catalyst composition in a bulk scale. According to thermal chemical deposition method, the form of metal oxide or metal hydroxide of Mg, Al and Si has been used as catalyst support materials. In case of silicon, high crystalline type of SiO2 has been used in a thermal chemical deposition method.
In the course of researching the catalyst composition for preparing carbon nanotube, the inventors of present invention found that the binding property of the transition metal with the support material and the control of particle size of a catalyst composition are detrimentally affected by the heat treatment step performed at high temperature under atmosphere when using a crystalline material as a support.
According to the conventional method for preparing a catalyst composition, the heat treatment step performed over 600° C. under atmosphere as well as the additional heat treatment for reducing the catalyst composition to enhance its activity has been required before the main decomposition reaction of the hydrocarbon on the catalyst particle. In view of a carbon nanotube manufacturer, it is desirable that the number of heat treatment steps is reduced and the hydrocarbon having low decomposition temperature such as ethylene is used as a carbon source for preparing carbon nanotube having less than 20 nm of diameter. However, it is still required to develop a catalyst composition which provides a high productivity and catalytic activity. Further, the high preparation cost of carbon nanotube prohibits the carbon nanotube from being commercially available in a bulk scale.
To overcome the above mentioned problems, the inventors of present invention have developed a catalyst composition for preparing carbon nanotube. We found that the carbon nanotube having less than 20 nm of diameter can be prepared without an additional heat treatment for the reduction of a catalyst composition before the decomposition reaction of hydrocarbon when amorphous silicon is used as a metal support material in a catalyst composition. It is further found that the productivity of carbon nanotube is also enhanced by selectively using other support materials together with amorphous silicon.
Finally, we have developed a catalyst composition which has consistent homogeneity between the transition metal particle and the support material by introducing an amorphous type of support material. We have further developed a preparation method for carbon nanotube, wherein the control of size of diameter, the control of the number of wall and length of carbon nanotube are accomplished by using the developed catalyst composition. In addition, a high catalytic yield is obtained even without using hydrogen gas as a reducing agent at the time of catalytic reaction of hydrocarbon. Ultimately, we have developed a preparation method for carbon nanotube having the following characteristics: the diameter of carbon nanotube is 5˜20 nm; the number of wall is less than 15; the apparent density of carbon nanotube is 0.03˜0.08 g/cc; and the carbon conversion yield is in the range of 40˜80%.